Dictionary mutations code?

["John R. Smith" <mmu6culus () gmail com>]

I am searching for scripts/programs that can take a word and generate a 
specific type of mutations from it.

E.g. taking the input word "GreenBay" one type of mutated output would 
be a list of all words where some of the applicable letters are replaced 
by the "elite" speech:

6reenBay
...
Gr3enBay
Gr33nBay
...
GreenB@y
...
6r33nB@y
etc.

Are there such scripts/programs already available in public domain?
Unix command-line would be ideal, but even windows 
DOS/Powershell/VBS/... are OK, too.

Clearly, with enough time, one can write something along these lines in 
Perl/shell/etc. I am trying to stay moderately lazy and hopefully just 
reuse a piece of code or pipe into a standalone binary that already does 
this (and other) mutation(s).

Thank you in advance for any advise,
-John

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